Scanning devices are used, for example, for direct exposure of printed circuit boards, where they scan a light bundle over a plane target surface along a scan line in a scan direction, while the target surface is moved in a forward feed direction running perpendicular to the scan direction. A generic scanning device such as that disclosed in WO 99/03012 A1 includes a light source, a scan unit, optics arranged upstream of the scan unit (pre-scan optics) to shape and guide a bundle of beams coming from the light source to the scan unit, and optics which are arranged downstream of the scan unit (post-scan optics) to shape and guide the beam bundle deflected by the scan unit onto a target surface.
According to an embodiment example of the above-cited WO 99/03012 A1, the pre-scan optics include a cylindrical mirror and a bicylindrical, refractive element and image the light source into the post-scan optics so as to provide the anamorphic characteristics of the beam bundle required by the post-scan optics. A deflecting mirror, which is arranged upstream of the scanning device, and an input mirror merely serve to deflect the beam, or to deflect and couple the beam into the scanning device, formed by a polygon mirror, at an input angle in the cross scan plane (under a sagittal input angle in the cited reference) formed by the beam axis of the beam bundle and the surface normal to the polygon mirror facets of the polygon mirror.
The polygon mirror facets reflect the beam bundle in the cross scan plane at an angle of reflection equal to the angle of incidence into the post-scan optics.
The post-scan optics are an F-theta objective in which there can occur various types of distortion which it is necessary to minimize. This distortion includes scan bow and trapezoidal distortion.
Scan bow is a curvature of the scan line relative to the scan plane which occurs in every F-theta objective and which increases with increasing distance of the image location from the optical axis of the post-scan optics.
Trapezoidal distortion is known in projector arrangements when the image is to be projected at various heights on a projection surface. The image is then typically wider at the upper edge of the image than at the lower edge of the image. This type of distortion arises when the object and/or the image are/is not arranged perpendicular to the optical axis of the projection objective. In an F-theta objective, the input angle and possibly a centered arrangement of the post-scan optics with respect to the input angle are determining factors in this respect.
Therefore, it is suggested for both arrangements of a scanning device according to the above-cited WO 99/03012 A1 that the post-scan optics downstream of the polygon mirror be arranged so as to be centered with respect to the polygon mirror, i.e., that the optical axis of the post-scan optics be perpendicular to the rotational axis of the polygon mirror so that the beam bundle reflected by the polygon mirror enters the post-scan optics at an angle to the optical axis of the post-scan optics equal to the angle of incidence and is imaged on the target object in an image point which lies in a plane including the optical axis. A centered arrangement of the post-scan optics with respect to the polygon is the arrangement having the least distortion. At the same time, however, it results in a fairly bulky construction in case of larger incident angles of the incident beam bundle.
According to one construction, the post-scan optics include, arranged in beam direction, a spherical meniscus and a plane cylindrical lens, which together form a doublet, and a spherical cylindrical lens. Comparable to the corrective objective of a scanning device according to the invention, the three optical elements mentioned above have the object of eliminating or minimizing the image aberrations that cannot be compensated by the subsequent mirror arrangement for generating a telecentric beam path. The subsequent mirror arrangement includes a first spherical mirror and a second spherical mirror and reflects the beam bundle via a spherical cylindrical lens into the focal plane of the post-scan optics comprising all of the elements mentioned above.
A scanning device according to the above-cited WO 99/03012 A1 is unsuitable especially for large scan lengths (i.e., generally, a long focal length of the post-scan optics in scan direction). A particular disadvantage consists in the large constructional size of the mirror arrangement owing to the first spherical mirror which has approximately twice the length of the scan line to be generated.